You can only import changes to stories from a previously exported list of stories, saved as excel-file.
When you rename the stories in Excel, the stories will get updated, as expected. They do not get added. Even if you change the story-id, when importing, you will get the message “ story <your storynumber> 'First story…' does not exist (anymore). “
So it is only possible to change existing stories. Adding new stories other than from within Sprintr, is possible, there is an API for that: https://docs.mendix.com/apidocs-mxsdk/apidocs/stories-api#2-2-microflow-createstory. But that is probably beyond the scope of the RapidDeveloper-path.